Knee noises may be normal in the knee or may signify problems, depending upon the nature and trigger for the noise.
Crepitus is a fine crackling feeling, which one can feel through your hand on your knee while bending and straightening the knee. It can also be audible. Sometimes it is less of a sound and more of a feeling. It is not specific of anything but really just a sign that the knee is not fully healthy, and is commonly felt with all types of arthritis. Experts suggest that crepitus is the result of fine air bubbles in the joint.
Clicking is a soft sound, often not particularly uncomfortable, and which may be associated with certain movements. It may arise from a damaged meniscus or a meniscus that is not quite standard in anatomy.
A patellar clunk is an occasional deep noise and feeling which may occur after some knee replacements. The clunk arises with bending and straightening the knee from a fibrotic nodule or lump of scar tissue at the top pole of the patella and the lower part of the quadriceps tendon.
Snapping is a sharp noise and discomfort as a tendon or a fold of joint lining (plica) snaps over a bony area.